Modern medical ventilators are designed to ventilate a patient's lungs with respiratory gas, and to thereby assist a patient when the patient's ability to breathe on his own is somehow impaired. As research has continued in the field of respiration therapy, a wide range of ventilation strategies have been developed. For example, pressure assisted ventilation is a strategy often available in patient ventilators and includes the supply of pressure assistance when the patient has already begun an inspiratory effort. With such a strategy, it is desirable to immediately increase the pressure after a breath is initiated in order to reach a target airway pressure for the pressure assistance. This rise in pressure in the patient airway which supplies respiratory gas to the patient's lungs allows the lungs to be filled with less work of breathing by the patient. Conventional pressure assisted ventilator systems typically implement a gas flow control strategy of stabilizing pressure support after a target pressure is reached to limit patient airway pressure. Such a strategy also can include programmed reductions in the patient airway pressure after set periods of the respiratory cycle in order to prepare for initiation of the next patient breath.
Conventional ventilators typically provide breath inhalation support at regular intervals, or at intervals triggered by a patient's spontaneous inspiration effort. The method of controlling the gas flow requires actuation of a gas flow valve. Errors in the delivery of gas flow as compared to the desired gas flow at the appropriate time can occur due to lag time between the onset of patient inspiratory effort and actual valve response time, regulator response, and valve gain variations. Although typical flow controllers may utilize a feedforward flow control gain component and various types of feedback error correction, such as proportional, integral, and derivative error feedback control, to compensate for real time disturbances that occur in the system, such systems frequently have difficulty in correcting for any sustained errors that occur periodically in the system.
Another method of controlling the gas flow rate has been the use of variable speed blowers or fans. The speed of such blowers can be rapidly increased or decreased to impart a desired rate of flow. This allows greater flexibility in controlling each inspiration and exhalation. The rapid rate of change in the gas flow allows the ventilator to vary the rate of flow multiple times or even continuously within the time span of a single breath. Such a ventilator can gently respond to the patient's initiation of inspiration and exhalation only when such responses are carefully monitored. Furthermore, a ventilator needs to make rapid and repeated adjustment to respond accordingly. A blower based ventilator is capable of such gas flow control.
Conventional flow control in a blower based ventilation system uses an electronic feedback controller to control the gas flow rate. As an example, a target flow rate can be input into a ventilator's electronic interface to initiate the control function. An electronic motor controller may be configured to control the speed of a motor attached to the blower or fan. Under ideal conditions, the speed of the motor determines the gas flow rate at the blower outlet. However, the static pressure at the blower outlet will affect the actual flow rate. Variations in the static pressure may generate variations in the actual flow rate even though the speed of the motor remains constant.
To account for variations in the static pressure, conventional blower based ventilators have relied upon an electronic feedback controller connected to an airflow transducer to measure the actual airflow generated by the blower. Such feed-back controllers have relied upon the proportional-integral-derivative method of control. This method is effective only under conditions where the static pressure does not change significantly. Unfortunately, in the environments experienced by medical ventilators, the static pressure may change significantly. Moreover, while blowing gas into a patient's lungs, the static pressure (experienced in this case as the back pressure created by the lungs) varies over time. This may ultimately cause large deviations from the target flow rate.
Therefore, despite the ability to rapidly vary the actual flow rates, conventional blower-based ventilator systems have been unable to accurately maintain a target flow rate. This is due, in part, to a control system that must continually adjust to an actual flow rate which varies over time. Such control systems continually change the motor speed to alter the actual flow rate which is also continuously changing in response to changes in the back pressure of the lungs. At worst, this method will produce an unstable control system which progressively deviates from the target flow rate. At best, this method will produce a minimally stable control system which continuously oscillates about the target flow rate.
What has been needed and heretofore unavailable in a blower based medical ventilator in a control system that accurately compensates for environmental pressures to stably maintain a target flow rate. The present invention fulfills this and other needs.